Copper-Catalyzed Covalent Nanostructure Synthesis on Atomically Flat Highly Oriented Pyrolytic Graphite (HOPG)

Abstract

On-surface Ullmann-type coupling has been widely employed for constructing atomically precise carbon-based nanostructures, yet its implementation remains largely restricted to metal substrates, where strong molecule–substrate interactions perturb intrinsic electronic properties and hinder further device integration. Extending on-surface reaction to chemically inert surfaces such as highly oriented pyrolytic graphite (HOPG) is fundamentally challenged by the competition between reaction activation and molecule desorption under ultrahigh vacuum (UHV) conditions. Here, we demonstrate classical Ullmann coupling on HOPG catalyzed by exogenous copper (Cu) atoms. Two brominated precursors, 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (pTBPT) and 5,10,15,20-tetra(4-bromophenyl) porphine (TBrPP), are employed to investigate how exogenous Cu regulates the critical competition between molecular desorption and reaction activation. For pTBPT, scanning tunneling microscopy (STM) reveals the formation of uniform and well-ordered oligomeric structures at 350 °C, whereas in the absence of Cu, molecules desorb without reaction. Synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and density functional theory (DFT) calculations further indicate that Cu atoms simultaneously promote molecular anchoring and reduce the activation barrier for dehalogenative coupling. As a complementary system, STM and XPS analysis of TBrPP reveals a critical competition between molecular desorption and dehalogenative coupling. While HR-XPS reveals in situ metalation at room temperature, STM imaging confirms that complete molecular desorption occurs at 200 °C, preventing subsequent dehalogenative coupling. These findings provide mechanistic insights into how transition metal mediates the interplay between surface reaction and desorption, while offering a strategy for the fabrication of structurally defined covalent nanostructures on electronically decoupled platforms.